1. Field of the Invention
The present invention relates to an image pickup apparatus, and in particular, to an image pickup apparatus being characterized by a processing part of an image pickup signal in a long time exposure mode.
2. Related Art Statement
Image pickup apparatuses each having an imaging device outputting a subject image as an electric signal by performing photoelectric conversion of the subject image have been widely used in fields such as TV cameras and electronic endoscopes. In such an image pickup apparatus, a subject image is photoelectrically converted and is imaged by an imaging device storing and transferring electric charges.
Regarding such an image pickup apparatus, various apparatuses are proposed. The apparatuses each have sensitivity enhancement means with which it is possible to obtain an excellent subject image by enhancing sensitivity through changing an exposure time when the subject image is dark.
For example, an image pickup apparatus proposed in Japanese Unexamined Patent Publication No. 6-326917 has the following structure and features. Plural field period is set as an electric charge storage time of an imaging device by imaging-device drive means. Furthermore, when the apparatus reads a picture signal from the imaging device every plural field period, picture signal calculation control means performs predetermined calculations for a picture signal outputted from the imaging device and a picture signal stored in picture signal storage means. Moreover, the picture signal calculation control means outputs the picture signal every period that is shorter than the plural field period. Therefore, the apparatus can easily enhance sensitivity with little degradation of an S/N ratio and can obtain an excellent subject image, which has natural motion, even if the sensitivity is enhanced and the subject moves.
The image pickup apparatus proposed in Japanese Unexamined Patent Publication No. 6-326917 picks up a subject image with changing an exposure time from {fraction (1/60)} sec to {fraction (1/30)} sec through changing a mode from a normal exposure mode to a long time exposure mode if the subject becomes dark and an image thereof achieves a certain luminance. Nevertheless, since the exposure time is changed from {fraction (1/60)} sec to {fraction (1/30)} sec in this manner, the luminance of an image obtained sharply changes, and hence the image seems to be unnatural. This is a problem of this apparatus.
The invention provides an image pickup apparatus with which it is possible to obtain a natural image without disharmonies by continuously changing luminance in the long time exposure mode.
The present invention also provides an image pickup apparatus with which it is possible to obtain a natural image without disharmonies by continuously changing luminance in the long time exposure mode even in regard to nonlinear image processing.
An image pickup apparatus of the present invention comprises: an imaging device photoelectrically converting a subject image and outputting the subject image as an image pickup signal every field period; imaging device drive means for obtaining the image pickup signal by controlling an exposure time and driving the imaging device so that an electric charge storage period of the imaging device can be changed; field delay means for delaying the image pickup signal from the imaging device by one field; and picture signal generation means for generating a picture signal, having predetermined luminance, from a field of image pickup signal from the imaging device and an image pickup signal delayed by one field from the field delay means. The image pickup apparatus makes it possible to obtain a natural image without disharmonic feeling by continuously changing luminance in the long time exposure mode through the field delay means for delaying the image pickup signal from the imaging device by one field and the picture signal generation means for generating a picture signal from a field of image pickup signal from the imaging device and the image pickup signal delayed by one field from the field delay means.
Other features and advantages of the present invention will be sufficiently apparent from the following description.